1. Field of the Invention
This invention relates to an optical information recording medium, and particularly to a reproduction-only optical information recording medium capable of recording and/or reproduction therein by using a recording/reproducing apparatus for magneto-optical recording medium.
2. Description of the Related Art
As the optical information recording media, the compact disc (CD) and the laser disc (LD) have already been prevailed broadly. These compact disc and laser disc are reproduction-only optical discs each of which has a reflective layer and, if necessary, a protecting layer on a substrate having pits and lands corresponding to information signals formed in advance, and from which pre-recorded information signals are reproduced. A material of high reflectance, that is low absorptance, such as Al or Au is used for the reflective layer.
Meanwhile, the magneto-optical disc has been developed recently as an optical information recording medium in which information is writable and deletable, in contrast to the above-mentioned reproduction-only optical disc. The magneto-optical disc has a recording section formed by stacking a recording magnetic layer having an axis of easy magnetization perpendicular to the film surface and large magneto-optical effect (such as a rare earth-transition metal alloy amorphous thin film), a reflective layer and a dielectric layer on a substrate. The magneto-optical disc of such structure having a small diameter of 64 mm, that is, a so-called mini disc, contained in a disc cartridge is now prevailing.
Meanwhile, in order to improve prevalence and practicality of both the reproduction-only optical disc and magneto-optical disc systems, it is preferred that reproduction can be carried out with the reproduction-only optical disc by the recording/reproducing apparatus for magneto-optical disc. However, the reproduction-only optical disc having the reflective layer of Al film or Au film has an extremely high reflectance of 70% or higher to a laser light, in comparison to the reflectance of magneto-optical disc of 15 to 25%. Therefore, reproduction of signals from the reproduction-only disc cannot be carried out by the recording/reproducing apparatus for magneto-optical disc.
Thus, a dye-containing layer composed of a dye material and a polymeric material is used instead of the Al film or the Au film, as the reflective layer. For instance, an optical disc using a cyanine dye as the dye material has a reflectance of 15 to 25%, similar to that of the magneto-optical disc, and is therefore expected to be promising as a reproduction-only optical disc applicable to the recording/reproduction apparatus for magneto-optical disc.
However, since the optical, disc of low reflectance using the dye-containing layer does not have a protection film, which is provided on the optical disc of high reflectance using the Al film, the surface of the dye-containing layer may be damaged or worn in contact with foreign matters.
In the optical disc having the dye-containing layer as the reflective layer, a readout light is reflected on the boundary between the substrate and the dye-containing layer and on the boundary between the dye-containing layer and the air, causing multiple interference, which is utilized for reproducing signals. Therefore, if the dye-containing layer is damaged or worn, turbulence is generated in the multiple interference of the light, disturbing reproduction of signals.
Also, since the dye-containing layer composed of the dye material and the polymeric material is difficult to dissolve in a solvent once it is hardened, recovery of compositions for recycling of used optical discs is difficult. Even though compositions of the dye-containing layer are recovered, the reflective layer formed by recycling the compositions has signal property and damage resistance inferior to those of the original reflective layer.